1. Field of the Invention
The present invention relates to an electrostatic image developer which is employed in electrophotographic methods, electrostatic recording methods, and electrostatic printing methods.
This application is based on Japanese Patent Application No. Hei 10-271250 filed in Japan, the content of which is incorporated herein by reference.
2. Description of Related Art
Various electrophotographic methods have been disclosed in, for example, U.S. Pat. No. 2,297,691, Japanese Patent Application, Second Publication No. Sho 42-23910, and Japanese Patent Application, Second Publication No. Sho 43-24748; commonly, an electrostatic latent image is formed on an electrostatic latent image bearing medium such as a photoconductive photosensitive medium or the like by means of charge or light exposure, and then this electrostatic latent image is developed using a toner composition containing a colorant in a binder resin, and the resulting toner image is transferred to a support medium such as transfer paper or the like and fixed, and a visible image is thus formed.
Furthermore, a large number of developing methods are known among electrophotographic methods; these can be broadly classified into two-component developing methods which employ, as a developer, a mixture of toner and a carrier comprising microparticles (20-500 micrometers) of, for example, iron powder, ferrite, nickel, glass, or the like, and single-component developing methods which employ a developer comprising only toner.
Representative examples of the two-component developing methods include the cascade method disclosed in U.S. Pat. No. 2,618,552, and the magnetic brush method disclosed in U.S. Pat. No. 2,874,063. In these methods, functions such as the agitation, conveyance, charging, and the like of the developer are apportioned to the carrier, so that the functional separation between the carrier and the toner is made clear. For this reason, the control of the charge of the toner and the formation of the developer layer are comparatively easy, and the method is capable of high speed, so that it is presently widely employed.
In recent years, in concert with the development of the information society, there have been increasing requirements for improvement in the quality of printed images, increase in recording speed, greater density, long term storage stability, and the like, in a variety of fields such as electrophotography, electrostatic recording, and electrostatic printing, and there has been a great desire for improvements in the characteristics of toner for recording electrostatic latent images on non-printing media. In particular, in toners employed in two-component developers which are used in high speed printing, the strength of the toner with respect to friction with the carrier, and stable fixing behavior in a broad range of temperatures in heat roller fixing methods, are important characteristics, and these characteristics are very frequently dependent on the characteristics of the binder resin which is employed in the toner composition.
Among single-component developing methods, for example, U.S. Pat. No. 4,336,318 discloses a magnetic single-component developing method for conducting development using an electrically insulating magnetic toner. In this method, a charge is introduced into the toner as a result of the frictional charge between the toner particles and the toner-bearing medium and the toner thin film forming member, or as a result of the frictional charge between the toner particles themselves, and an electrostatic latent image is deposited on the photosensitive medium.
This developing method does not employ a carrier, and does not require a device for controlling the mixing ratio between the carrier and the toner, so that it is advantageous in that the developing apparatus has a small size.
In this method, in order to form the magnetic brush of the toner on a metal sleeve, it is necessary to provide the appropriate magnetic characteristics to the toner itself, and for this reason, it is absolutely necessary that magnetic materials such as magnetite, ferrite, and the like be contained in the toner. The amount of these magnetic materials contained varies somewhat depending on the developing conditions and type of materials; however, an amount within a range of 30-60% by weight is common.
Accordingly, the fact that the proportion of binder resin which is contained in the toner powder is small in comparison with that in a two-component developing toner is disadvantageous from the point of view of the fixation properties. As a result of these circumstances, the development of a binder resin which is capable of exhibiting sufficient fixation properties even when the proportion of binder resin contained is low in a magnetic single-component developing method has been keenly desired.
On the other hand, in order to solve the problems in the single-component developing method employing magnetic toner in this manner, a non-magnetic single-component developing method has been proposed which does not require that the toner have magnetic properties. Various apparatuses have been devised employing such a method; in many of these, toner is deposited on a developing sleeve or the like by means of static electric power, and the toner is then conveyed to a latent image surface and developed.
In non-magnetic single-component developing methods, an electrophotographic powdered toner is employed which contains, as required components, a binder resin, a colorant, and a charge control agent; the binder resin employed in this method is, like the binder resin employed in other developing methods, required to have stability with respect to static electricity, durability during continuous printing, and stable fixing behavior over a wide range of temperatures.
The binder resins for toner which have been investigated include, for example, polystyrene, styrene-acrylic ester copolymer, styrene-butadiene copolymer, polyester, epoxy resin, polybutyral, xylene resin, coumarone-indene resin, and the like, and various proposals have been made for the design of such resins depending on the use thereof.
Generally, characteristics which are required in binder resins include a variety of characteristics such as charge, fixation characteristics, and the like; in particular, in binder resins which are employed in toner used in heat roller fixation, an improvement in the fixation properties with respect to the transfer paper, and the anti-offset properties with respect to the heat roll is required. In heat roller fixation, the toner particles which are electrostatically deposited on the transfer paper are fused by means of a passage through the pressurized and heated hot rollers, and are fixed on the transfer paper. When the surface temperature of the rollers is too low at this time, the toner particle layer as a whole is not sufficiently heated, and only that surface which comes into contact with the heated rollers is softened and deposited on the heated rollers. The toner on the transfer paper side is not softened, so that no adhesion force is produced, and as a result, almost all of the toner layer on the transfer paper moves to the fixing roller without being fixed on the transfer paper. This is termed a cold offset.
On the other hand, when the temperature on the roller surface is too high, the viscosity of the molten toner decreases, and in concert with this, the internal cohesive force of the molten toner layer also decreases precipitously and becomes less than the force of adhesion to the heating roller. As a result, the molten toner layer is ruptured and moves both to the transfer paper and to the fixing roller. This is termed the hot offset, and is a cause for the contamination of the heating roller. The toner deposited on the heat roller is retransferred to the transfer paper and causes contamination of parts other than the image, so that the quality of printing declines.
What is meant by anti-offset properties is the ability of the toner at certain temperatures to avoid giving rise to cold offset or hot offset; the binder resin employed in the toner is required to have offset resistance properties over a wide range of temperatures, and is required to have superior fixation properties.
In order to obtain the above object, a number of designs have been proposed; among these, in order to maintain the viscoelasticity during heating and melting, or to suppress the changes in viscosity resulting from fluctuations in temperature, techniques have been considered in which mechanisms are employed to broaden the molecular weight distribution, to provide crosslinking structures, and to make use of rubber elastic material. In the investigations to date, it has been widely known that polyester resin may be employed as a resin for heat roller fixation. This is the case in, for example, Japanese Patent Application, Second Publication No. Sho 52-25420, Japanese Patent Application, Second Publication, No. Sho 53-17496, Japanese Patent Application, Second Publication No. Sho 55-49305, Japanese Patent Application, First Publication No. Sho 55-38524, Japanese Patent Application, First Publication No. Sho 57-37353, Japanese Patent Application, First Publication No. Sho 58-11952, and the like. However, in the conventionally proposed inventions, a polyester resin which sufficiently achieves the anti-offset properties over a broad range of temperatures and the fixation properties which are currently sought has not been obtained.
In Japanese Patent Application, First Publication No. Sho 62-295068, an electrostatic image developer is disclosed which is characterized in that the following are employed as the electrostatic static image developing agent employing a polyester resin as the binder resin:
(1) an epoxy compound having a valence of 5 or more, PA1 (2) A polybasic acid compound having a valence of 2 or more selected from a group consisting of polybasic acid and/or acid anhydride and/or lower alkyl esters thereof, and PA1 (3) a polyvalent alcohol having a valence of 2 or more, and in that the polyester resin is obtained by first reacting components (2) and (3) and then reacting component (1). PA1 (1) an epoxy compound having a valence of 5 or more, PA1 (2) A polybasic acid compound having a valence of 2 or more selected from a group consisting of polybasic acid and/or acid anhydride and/or lower alkyl esters thereof, and PA1 (3) a polyvalent alcohol having a valence of 2 or more, by reacting components (1)-(3) all at once, or by first reacting components (1) and (3) and subsequently reacting component (2), or by first reacting components (1) and (2) and subsequently reacting component (3); the present invention also provides an electrostatic image developer which is characterized in being an electrostatic image developer comprising at least a magnetic carrier and colored resin particles containing at least a binder resin, a colorant, and a charge control agent, wherein the binder resin is a polyester resin obtained using:
However, a resin which is resistant to the shear within the developing apparatus, in particular during high speed printing, and which has sufficient fixing strength at low temperatures, and which furthermore has anti-offset properties at high temperatures, has not yet been discovered.
Additionally, in order to provide releasing properties from the heat roller during fixing, and in order to prevent the generation of offset, techniques have also been developed in parallel in which a releasing agent is included in the toner. To date, attention has centered on synthetic waxes such as polypropylene wax, polyethylene wax, and the like; however, examples have been disclosed in which a natural wax, such as montan wax, carnauba wax, rice wax, and the like have been employed, in Japanese Patent Application, First Publication No. Hei 1-238672, Japanese Patent Application, First Publication No. Hei 3-5764, and Japanese Patent Application, First Publication No. Hei 5-119509.
With respect to charge control agents, as well, various such agents have been considered, and a positive charge charge control agent or a negative charge charge control agent is selected depending on the development method and the polarity of the photosensitive medium. For example, nigrosine dyes and quaternary ammonium salt-compounds and the like are known as charge control agents which may be employed in positively charged toner which is employed in machines using high speed and highly durable selenium photosensitive media. Examples in which such positive charge charge control agents are used singly or in combination are disclosed in, for example, Japanese Patent Application, First Publication No. Hei 1-259371, Japanese Patent Application, First Publication No. Hei 3-7948, Japanese Patent Application, First Publication No. Hei 5-119509, and Japanese Patent Application, First Publication No. Hei 10-246991.
However, a discovery which exhibits all the characteristics required in the developing methods described above has not been disclosed in any of the referenced publications.